Future wireless sensor networks will comprise sensor nodes which occupy a volume of typically a few cm3. The scaling down of batteries for powering these sensor nodes faces technological restrictions as well as a loss in storage density. Replacing or recharging the batteries at regular time intervals, for example every month, would be very expensive or even impossible, for example in distributed or implanted systems. Energy scavengers based on the recuperation of wasted ambient energy are a possible alternative to batteries. Several scavenger concepts have been proposed, based on the conversion of light energy (using solar cells), thermal energy (thermoelectric generators), pressure energy, or kinetic energy.
Kinetic energy scavengers convert energy in the form of mechanical movement (e.g. in the form of vibrations or random displacements) into electrical energy. For the conversion of kinetic energy into electrical energy, different conversion mechanisms may be employed, for example based on piezoelectric, electrostatic, or electromagnetic mechanisms. Piezoelectric scavengers employ active materials that generate a charge when mechanically stressed. Electrostatic scavengers utilize the relative movement between electrically isolated charged capacitor plates to generate energy. Electromagnetic scavengers are based on Faraday's law of electromagnetic induction and generate electrical energy from the relative motion between a magnetic flux gradient and a conductor.
Electrostatic energy conversion is based on a variable capacitance structure that is driven by mechanical vibrations and oscillates between a maximum capacitance and a minimum capacitance. Movement of a seismic mass resulting from external vibrations is translated into a change of the capacitance and thus into a change of the charge on the capacitor. This results in an electrical current through a load circuit, and thus a conversion of kinetic energy into electrical energy. In micromachined electrostatic scavengers the relative movement between electrically isolated capacitor plates is obtained by providing a fixed electrode and a movable electrode (i.e. movable relative to the fixed electrode). Often the movable electrode and the fixed electrode comprise a plurality of shallow capacitor plates in parallel, called fingers. The fingers of both electrodes may be interdigitated or not. A seismic mass may be attached to the movable electrode.
The fixed electrode and the movable electrode may be located in a same plane (‘in-plane’). Relative movement between the capacitor plates may then comprise changing the overlap area of the fingers (in-plane variable overlap capacitor) or changing the gap between the fingers (in-plane gap-closing capacitor). Alternatively, the fixed electrode and the movable electrode may be located in different planes (‘out-of-plane’), with a spacing or gap in between both electrodes. Relative movement between the capacitor plates may comprise changing the gap between two large plates (out-of-plane gap closing capacitor) or changing the overlap between a plurality of fingers (out-of-plane variable overlap capacitor).
The out-of-plane variable overlap approach allows a larger displacement of the seismic mass and a larger capacitance change as compared to the in-plane overlap approach, and offers a reduced susceptibility to the pull-in effect as compared to gap-closing capacitors. The fixed electrode and the movable electrode of an out-of-plane variable overlap capacitor form a plurality of parallel-plate capacitors that are connected in parallel.
In prior art systems such an out-of-plane variable overlap structure is fabricated based on at least two substrates, as e.g. reported by G. Altena et al. in “Electrostatic energy scavengers for wireless autonomous sensor nodes”, Smart Systems Integration 2007, proceedings of the 1st European Conference and Exhibition on Integration Issues of Miniaturized Systems—MEMS, MOEMS, ICs and Electronic Components, pages 359-366, March 2007. FIG. 1 shows a cross section of such a prior art out-of-plane variable overlap capacitor structure. The structure comprises a fixed electrode comprising a plurality of fixed fingers 11 formed on a first substrate 10, for example a glass substrate or any other suitable substrate. It further comprises a movable electrode comprising a plurality of movable fingers 21 and a seismic mass 22 physically attached to the movable electrode. The movable fingers 21 and the seismic mass 22 are made from a second substrate 20, e.g. a silicon substrate or any other suitable substrate. The movable electrode of the capacitor is for example bulk micromachined together with the mass 22 and the suspensions (not illustrated in FIG. 1). The first substrate 10 and the second substrate 20 are adhesively bonded to each other with an adhesive 31, for example with a photosensitive BCB layer. Often a third substrate is added (not illustrated in FIG. 1), bonded to the second substrate 20 at a side opposite to the side where the first substrate 10 is bonded. This third substrate can be used to package the MEMS structure, possibly in vacuum to reduce damping losses. The third substrate can also be used as a means to polarize the capacitor, either using an electrode or an electret.
Fabrication of such a prior art out-of-plane variable overlap capacitor is relatively complex, requiring at least two substrates 10, 20, at least four patterning steps (two per substrate) and a wafer bonding step with good alignment between the fixed fingers 11 on the first substrate 10 and the movable fingers 21 on the second substrate 20. The size of the gap 32 between the fixed electrode and the movable electrode of the capacitor can be controlled down to 1 micrometer by tuning the thickness of the adhesive 31. Variations in the gap 32 can occur, in case of thickness non-uniformities in the adhesive layer, e.g. resulting from spincoating the adhesive on a substrate with topography. The performance of the capacitor may depend on the accuracy of the alignment of the wafer bonding procedure, mainly on the accuracy of the rotational alignment. Furthermore, the yield depends on the amount of bonding defects after wafer bonding.